Photoresists are light sensitive compositions that produce chemical changes in the compositions after exposure to light. These changes allow chemical differentiation between exposed and unexposed areas using a developer, typically an aqueous alkali solution. When a mask having a pattern of transmissive and reflective areas is used, this chemical differentiation produced relief images. The produced images allow transfer of the image into underlying substrates for electronic device manufacture or to other materials in a printing process.
In recent years, advanced resists comprise a photosensitive acid generator (PAG), a polymer matrix, and sometimes additives. Acid sensitive groups that inhibit dissolution of the polymer in developer are present on the additive or on the polymer. Upon exposure, the acid generated from the PAG catalyzes the removal of the acid sensitive group that is inhibiting dissolution in the developer. One acid proton will catalyze reaction at many different sites, so the technique is called “chemical amplification”. Photoresists of these type can be found, e.g. in U.S. Pat. No. 6,120,977, U.S. Pat. No. 6,136,504, U.S. Pat. No. 6,013,416, U.S. Pat. No. 5,985,522, U.S. Pat. No. 5,693,453, and U.S. Pat. No. 4,491,628.
Various classes of PAGs have been utilized in acid catalyzed photopolymer systems. Particularly favored are those that produce a sulfonic acid. Numerous types of PAGs have been demonstrated to produce sulfonic acids upon irradiation. Examples of some of the classes are shown below. Particularly favored are those PAGs of the formulae: where R is a perfluoroalkyl because of the strong acidity of the perfluoroalkyl sulfonic acids and other factors. Particularly preferred is perluorooctanesulfonic acid because of the low diffusion length. However, recently, the EPA has raised concerns regarding health and environmental impact of some long chain perfluorinated sulfonic acid salts (viz., perfluorooctanesulfonate). Shorter perfluoroalkyl chains sulfonic acids do not address the combination of health and performance concerns. As the perfluoroalkyl chain becomes shorter it is believed to increase the diffusion length. Triflic acid (R═CF3) has been shown to be volatile under some processing conditions, which can give poor results. In addition, the long-term viability and availability of other, shorter chain perfluorinated sulfonic acid salts is also in question due to the environmental and health issues. It is desirable to have sulfonic acids which have comparable acidity (i.e., pKa) and diffusion lengths to the larger perfluorinated sulfonic acids but without a heavily fluorinated alkyl chain directly attached to the sulfonic acid moiety.